The present invention generally relates to a sampling frequency down-converting apparatus, which makes it possible to effect a recording operation on a recording medium as the digital signals of the comparatively low sampling frequencies or to effect transmission operations among the digital picture apparatuses in spite of the increase in the required frequency zone from the signals of the NTSC system (hereinafter referred to as an NTSC signal) and the signal of the PAL system (hereinafter referred to as a PAL signal), in the signals of systems called an ATV (Advanced Television) system, an HD - MAC (High Definition - Multiplexed Analogue Components) system and so on especially even among the television signals, namely, of the next generation television system which is designed so that the aspect ratio 3:4 of the NTSC system of the current television signal system or of the PAL system may be made as sideways long as 9:16, and the resolution may be improved.
The NTSC signals only will be described hereinafter now that the same things may be said even about the PAL signals. Generally, in a digital picture apparatus such as a digital switcher handling the NTSC signals, a digital video tape recorder (hereinafter referred to as D - VCR) or the like, the NTSC analog signals are sampled at four times the color sub-carrier frequency Fsc, i.e., at 4 Fsc.
From Fsc .apprxeq.3.58 MHz, the sampling frequency becomes 4 Fsc .apprxeq.14.3 MHz. At this time, the frequency zone of the NTSC signals to be digitized is almost determined by a pre-low-pass filter (hereinafter referred to as an LPF) provided on the input side of an A/D converter. Normally, the flat portion of the frequency characteristics of the LPF is to approximately 5.5 MHz from 0 (direct current). Since the required frequency zone of the NTSC signals is 4.2 MHz, the NTSC signals are digitalized with a surplus by the sampling frequency of 4 Fsc.
An ACTV (Advanced Compatible Television) system is recently worth notice among an ATV system for improving the resolution with the aspect ratio of the above-described NTSC system some systems being made long sideways from 3:4 to 9:16.
The ACTV system only is described hereinafter now that the same thing can be said even about the other ATV systems. In the ACTV system, the field frequency and the color sub-carrier frequency are the same as in the NTSC system. But in the horizontal scanning frequency, there are a case where there is 15.75/1.00 1 kHz as that of the NTSC system, and a case where there is 31.5/1.00 1 kHz which is twice 15.75 so as to effect 525 sequential scannings for each one field. The former is called ACTV - E (Advanced Compatible Television - Entry) system, while the latter is called ACTV - 1.
The frequency zone of the ACTV - E system increases by the oblong portion and the horizontal resolution improvement portion as compared with that of the NTSC system. In order to have sideways long portion, the required frequency zone 4.2 MHz of the NTSC system increases as follows. EQU (16/9)/(4/3).times.4.2 MHz =(4/3).times.4.2 =5.6 MHz
In order to further improve the horizontal resolution as compared with that of the NTSC system, the ACTV - E system finally requires the flat frequency zone of 7 MHz.
Also, in the case of ACTV - 1 system, the zone increases twice to effect the sequential scannings with respect to the interlace scanning of the ACTV - E system, thus resulting in 14 MHz.
In a case of digitizing the signals (hereinafter referred to as ACTV signals) of such an ACTV system, in the sampling of the conventional 4 Fsc, the zone to be converted is approximately 5.5 MHz as described hereinabove and is insufficient. Therefore, it is impossible to serve a double purpose the digital picture apparatus of the 4 Fsc sampling for the conventional NTSC system use, which is the original purpose of the ACTV system. Thus, conventionally it has been considered to increase the sampling frequency to 5 Fsc or 6 Fsc. But they are both in a direction of increasing the data rate. The D - VCR for recording them reduces the recording time as compared with the NTSC system in the recording operation on the recording medium of the same capacity. In our time of longer hours' image transcription, it is a big task to be solved for the D - VCR which records the signals in the next generation television system.